Make SonataEasyExtends optional

[jordisala1991]

Subject

I am targeting this branch, because this is BC.

Part of: sonata-project/dev-kit#750

SonataEasyExtends is deprecated but it is used on many bundles, this makes it optional. This change is BC because if the user does not change anything it will continue using the deprecated code but with a message.

Changelog

### Changed
- SonataEasyExtendsBundle is now optional, using SonataDoctrineBundle is preferred
### Deprecated
- Using SonataEasyExtendsBundle to add Doctrine mapping information
### Removed
- Support for Symfony < 4.4

To do

• Update the documentation;
• Add an upgrade note.

[greg0ire]

Thanks @jordisala1991 !